An exhaust gas treatment system for diesel engines generally includes a diesel oxidation catalyst (DOC) and a diesel particulate filter (DPF) which is arranged downstream of the DOC. The DOC oxidizes and purifies hydrocarbon (HC) and carbon monoxide (CO) in exhaust gas, and oxidizes, among nitrogen oxides (NOx), nitrogen monoxide (NO) into nitrogen dioxide (NO2). The heat of catalytic reaction generated by the DOC increases the temperature of the DPF, and the high oxidizability of NO2 promotes combustion of particulate matters (PMs) deposited on the DPF. Since activity of the DOC is low immediately after start-up of the engine, zeolite is provided for the DOC as an HC trapping material to prevent HC from being exhausted without being purified.
On the other hand, a lean NOx trap catalyst (LNT catalyst) is also utilized in lean-burn gasoline engines or diesel engines to purify NOx. An NOx storage material in the LNT catalyst stores NOx when the air-fuel ratio of an exhaust gas is lean. A rich purge changes the air-fuel ratio in the engine to rich, and releases NOx and reduces NOx with unburned gas. An alkali metal or an alkali earth metal may be used as the NOx storage material. However, an alkali metal forms a glass phase on the grain boundaries of cordierite forming a catalyst substrate to reduce strength of the substrate. Therefore, an alkali earth metal is actually adopted in general, because it will not cause such a problem.
As disclosed in Patent Document 1, in an exhaust gas purifying catalyst for gasoline engines which has been proposed, an HC adsorbent layer containing zeolite and a catalytic metal layer containing an NOx storage material are stacked one upon the other on a monolithic support. This can adsorb HC and NOx simultaneously in the exhaust gas immediately after start-up of the engine. Release of HC and NOx and reaction between that HC and that NOx after activation of the catalytic metal can purify both HC and NOx.